Attempts have been made to improve the properties of printable substrates, such as base papers of printing papers, by different surface treatments of the substrates. Surface treatments can improve the properties even so that cheaper raw material can be used in the base paper without affecting the properties of the paper. It is, for example, possible to replace chemical pulp with mechanical pulp or increase the filler proportions of the pulp. Surface treatments may be used to improve the properties of paper, such as gloss or surface resistance or printability of the paper. Printability properties include among other things density, gloss of the print and smoothness of the print.
Water solutions having effective substances, such as starch or polyvinyl alcohol, added to the solutions, are often used in surface treatments. Various techniques to produce surface treatments are known. These include treatments done with a surface-size press or spray coater. One alternative is to spray the surface treating agent onto the surface of the substrate. Normally it is advantageous if the desired effect can be attained with the smallest amount of substance. As the surface treating agent dries, it forms a surface layer onto the substrate.
Different base papers and different printing techniques can require different surface treatments. Common printing methods include offset and gravure printing techniques.
In offset printing a smooth-surfaced printing plate is used, which plate has hydrophobic areas which repel water and hydrophilic areas which repel printing ink. Printing plate is soaked with water, which then adheres to the hydrophilic areas. After that the brayed printing ink adheres to the hydrophobic areas. Printing ink and water are then transferred onto an elastic roll from which they are transferred onto the surface of the paper and the print forms onto the paper. Many variations are known in offset printing, such as heat set offset and cold set offset.
In gravure printing a printing plate is used, which plate has point-like recesses carved onto it for the printing ink. Printing ink can be brought into the recesses by dipping the roll in printing ink and scraping the excess ink off the surface of the roll. When bringing the ink onto the surface of the paper, an electric field can be used in order to lift the printing ink from the recesses.
One surface treatment used to improve the printability of the paper is disclosed in US publication 2007/0107865. A mixture formed from alkyl ketene dimer as well as the copolymer of styrene and anhydride of maleic acid is used as a surface treatment chemical. A surface treatment chemical is used in connection with i.a. SC papers. Surface treatment is especially suited for paper used in gravure printing.
Hemicellulose is a known natural polymer. It can be found from different plants, such as trees. The hemicellulose composition varies depending on the plant and even on the parts of plant, and usually more than just one type of hemicellulose is present in the plant. Hemicelluloses are water-soluble and they easily dissolve into the water, for example when the wood raw material is processed during the manufacturing of pulp. The exiting waste water from the paper or cellulose factories usually contains a substantial amount of hemicellulose which strains the waste water facility of the factory.